Soft contact lenses are typically formed using a two-piece mold composed of a front mold half and a back mold half. The front and back mold halves are positioned so that they are spaced apart in a predetermined relationship. A monomeric material in liquid form is introduced in the gap between the front and back mold halves. The monomeric material, upon curing, forms the contact lens. The front and back mold halves are configured to shape the lens in a particular manner, so that the lens possesses optical properties that produce a desired vision correction in the end user.
The curing of the monomeric material is inhibited in the presence of oxygen. Hence, soft contact lenses are typically manufactured in an environment that is substantially free of oxygen. In addition, oxygen can permeate into the plastic material from which the front and back curves are formed, if the front and back mold halves are exposed to air. The oxygen needs to be removed from the front and back mold halves prior to use. The process by which the oxygen is removed is commonly known as “degassing.”
Front and back mold halves are typically manufactured by injection molding. The front and back mold halves are usually produced in batches, or lots, with each lot corresponding to a lens shape providing a particular vision correction. The lots may be stored after manufacture, until needed in the production process.
The lots of front and back curves are usually exposed to the ambient environment during storage. Hence, each lot needs to be degassed prior to use. Degassing is commonly performed by placing the front and back mold halves in a vacuum chamber, and subjecting the front and back curves to a relatively high vacuum, such as one Torr or greater, for a prolonged period such as eight to twelve hours.
Degassing using a vacuum chamber can be disadvantageous because additional lots cannot be placed in the vacuum chamber without interrupting any ongoing degassing operations. Interrupting a degassing operation before completion usually necessitates restarting the degassing operation from the beginning. Hence, multiple vacuum chambers, each with attendant start-up and operating costs, may be needed to conduct degassing operations on a substantially continuous basis.
The degassed front and back mold halves may be temporarily stored in nitrogen-filled containers until needed on the production line. These containers typically lose nitrogen through leakage and additional nitrogen may need to be supplied to each container on a continuous basis. The nitrogen supply line associated with each container is usually coupled to the container using relatively expensive fittings. Moreover, it can be difficult or impossible to determine whether an adequate nitrogen charge is being maintained in the container during storage.
Consequently, a need exists for a device and a method for degassing front and back mold halves on a substantially continuous basis, and for storing the degassed front and back mold halves in a cost-effective manner that minimizes the potential for the front and back mold halves to be exposed to oxygen.